iPhone 6s replacement Warning!

On older iPhones, the IMEI* is printed on the back of the iPhone. In iPhone 6s and later, the IMEI is on the SIM tray. Maybe that's a privacy feature, I don't know.
But the key thing is that if you're switching your SIM card to a new iPhone, it's possible to switch the SIM trays. That would leave you with the wrong IMEI if the iPhone can't start up.
By luck, I didn't make this mistake.